Disc drives read and write information along concentric tracks formed on discs. To locate a particular track on a disc, disc drives typically use embedded servo fields on the disc. These embedded fields are utilized by a servo subsystem to position a head over a particular track. The servo fields are written onto the disc when the disc drive is manufactured and are thereafter simply read by the disc drive to determine position. A servo system samples the position of the read/write head relative to a particular track at a particular sampling rate and adjusts the position of the head.
In a typical servo system, the actual position of the read/write head relative to a given track is sensed and compared to the desired position of the head. A position error signal (PES) indicative of the difference between the actual and desired positions is provided to a servo controller. Based on the value of the position error signal, the servo controller provides a servo control signal to a power amplifier that amplifies the servo control signal and provides it to a voice coil motor. The voice coil motor is coupled to an actuator that moves in response to the application of the amplified control signal to the voice coil motor. The actuator includes an actuator arm that holds the read/write head. In this way, the servo controller controls the positioning of the read/write head relative to a particular track on the disc surface.
Thus, a disk drive mechanical structure is composed of multiple mechanical components that are pieced together to form the final disk drive assembly. Each of these components has various resonant modes that if excited by an external energy source will cause the part to physically move (resonate) at the natural frequencies (resonance frequencies) of oscillation for the component in question. This movement can occur in a variety of different modes, for example, a bending mode, a twisting mode or a combination of the two. One component that contributes greatly to such resonant vibration is the actuator. If a component is highly undamped (i.e. the resonance is high amplitude, narrow frequency band) it will tend to oscillate with a minimal external driving energy. This oscillation results in physical motion of the data head, causing off track errors and potential fly height problems.
If resonances occur in a disk drive, they can severely limit drive performance, both in seek mode and track-follow mode. To obtain the optimal disk drive performance requires that there be no resonances present. However, this scenario is not physically possible. Every mechanical component has a natural frequency of oscillation. Nevertheless, it is desirable to reduce or minimize the resonances. Various schemes can be employed to damp the mechanical components and thereby decrease the amplitude of the resonant mode. Many such resonance-reducing techniques make use of information regarding the resonance characteristics of the disc drive mechanical structure. If the resonance characteristics vary greatly from one drive to the next among a production line of drives, the resonance characteristics data used by the resonance-reducing scheme are likely to be inaccurate. Thus, reducing the variance in the resonance characteristics from one drive to the next will increase the accuracy of the resonance-reducing scheme. Controlling the resonance natural frequency reduces vibration, which in turn allows the servo control loop to be tuned to the mechanical structure.
Present-day disc drive actuators are usually manufactured by casting or extrusion processes. The casting process involves placing a castable substance in a mold or form and allowing it to solidify. Extrusion consists of forcing a semisoft solid material, such as metal, through the orifice of a die to form a continuously formed piece in the desired shape of the actuator. Typically the resulting length of material is then cut into individual longitudinal sections, each corresponding to a single actuator. The placement of each cut thus defines the top of one actuator (on one side of the cut) and the bottom of another actuator (on the other side of the cut). Thus, the cross-sectional shape of the actuators, as viewed from above or below, is defined by the extrusion process.
The processes of casting and extruding actuators inherently have profile tolerances of 0.010 to 0.020 inches. These levels of tolerance result in significant variation in arm resonance from actuator to actuator. As track pitches have increased, the need for more precise control of actuator arm resonance frequencies has increased correspondingly, partially due to the tight tracking requirements and partially due to higher servo loop gain.
The present invention provides a solution to this and other problems and offers other advantages over the prior art.